Lithium ion secondary batteries have lighter weight and more excellent input/output characteristics than conventional secondary batteries such as nickel cadmium cells, nickel hydrogen cells and lead-acid batteries and have therefore been considered promising in recent years as power sources for electric vehicles and hybrid vehicles. Carbon materials are used as active materials in lithium ion secondary battery electrodes, and have been extensively studied with the aim of increasing lithium ion secondary battery performance (see Patent documents 1 and 2, for example).
Carbon materials used as negative electrode materials in lithium ion secondary batteries are generally classified as either graphite or amorphous. Graphite carbon materials have the advantage of high energy density per unit volume compared to amorphous carbon materials. For this reason, graphite carbon materials are widely used as negative electrode materials in lithium ion secondary batteries for cellular phones and laptop computers that are compact and require large service capacities. Graphite has a structure with layers of carbon atoms regularly arranged in a hexagonal mesh, and during charge-discharge, intercalation-deintercalation of lithium ions takes place at the edges of the hexagonal mesh.